In the development of electroluminescent elements utilizing organic materials (hereinafter referred to as organic EL element), the kind of electrodes was optimized for the purpose of improving the charge-injecting efficiency from the electrodes and an element in which a hole-transporting layer comprising an aromatic diamine and a light-emitting layer comprising 8-hydroxyquinoline aluminum complex are disposed as thin films between the electrodes has been developed (Appl. Phys. Lett., Vol. 51, p. 913, 1987) to bring about a noticeable improvement over the conventional elements utilizing single crystals of anthracene and the like. Following this, the developmental works of organic electroluminescent elements have been focused on their commercial applications to high-performance flat panels characterized by self luminescence and high-speed response.
In order to improve the efficiency of such organic electroluminescent elements still further, various modifications of the aforementioned basic structure of anode/hole-transporting layer/light-emitting layer/cathode have been tried by suitably adding a hole-injecting layer, an electron-injecting layer, and an electron-transporting layer. For example, the following structural variations are known: anode/hole-injecting layer/hole-transporting layer/light-emitting layer/cathode; anode/hole-injecting layer/light-emitting layer/electron-transporting layer/cathode; and anode/hole-injecting layer/light-emitting layer/electron-transporting layer/electron-injecting layer/cathode. The hole-transporting layer has a function of transporting the holes injected from the hole-injecting layer to the light-emitting layer while the electron-transporting layer has a function of transporting the electrons injected from the cathode to the light-emitting layer.
The disposition of the hole-transporting layer between the light-emitting layer and the hole-injecting layer enables one to inject a larger number of holes to the light-emitting layer by applying a lower electric field. Similarly, the disposition of the electron-transporting layer between the light-emitting layer and the electron-injecting layer enables one to inject a larger number of electrons by applying a lower electric field.
A large number of organic materials conforming to the function of these layered structures have been developed. However, none of the organic EL elements using such organic materials can be said to possess satisfactory properties as yet. The largest cause therefor is insufficient durability of the materials used, particularly, the lack of durability in the materials for the hole-transporting layer. If a nonuniform portion such as a grain boundary exists in any of the organic layers of an organic El element, the electric field would converge upon that portion and eventually cause deterioration or destruction of the element. For this reason, the organic layers are often used in an amorphous condition. Moreover, an organic EL element is an element of the current injection type and it may deteriorate under the influence of the Joule heat generated during driving when the glass transition temperature (hereinafter referred to as Tg) of the materials used is low. Hence, the materials are required to have high Tg. At the same time, it is necessary to improve the luminous efficiency of the element by improving the hole-transporting capability of the hole-transporting material used therefor.
The prior technical documents relating to this invention are listed below.    Patent literature 1: JP2851185 B    Patent literature 2: JP9-194441 A    Patent literature 3: JP8-100172 A    Patent literature 4: JP2002-203685 A    Patent literature 5: JP2001-126873 A    Patent literature 6: JP2001-39333 A    Patent literature 7: JP2001-114735 A    Patent literature 8: JP9-157643 A    Non-patent literature 1: Appl. Phys. Lett., Vol. 51, p. 913, 1987    Non-patent literature 2: Appl. Phys. Lett., Vol. 57, p. 531, 1990
A variety of hole-transporting materials, triphenylamine derivatives among many, are known for organic EL elements, but few of them are suitable for practical use. For example, N,N′-bis(3-methylphenyl)-N,N′-diphenyl-4,4′-diaminobiphenyl (hereinafter referred to as TPD) is reported in Appl. Phys. Lett., Vol. 57, p. 531, 1990, but this compound lacked heat stability and caused problems in service life when used in elements.
Other examples are N,N′-dinaphthyl-N,N′-diphenyl-4,4′-diaminobiphenyl (hereinafter referred to as NPD) reported in JP2851185 B and a derivative of NPD reported in JP9-194441 A; these compounds are described to show improvements over TPD in properties, but they were not yet satisfactory in the hole-transporting capability and heat resistance. Furthermore, attempts at structural modification to improve the heat resistance are reported; for example, modification of the biphenyl skeleton in JP8-100172 A, JP2002-203685 A, JP2001-126873 A, and JP2001-29333 A and increase in quantity of arylamine units in JP2001-114735 A. However, none of such materials showed high heat resistance, good stability of thin film, and high luminous efficiency at the same time.
As described above, the conventional organic EL materials for the hole-transporting and other functions of organic EL elements do not possess satisfactory properties suitable for practical use and there are expectations that the use of materials of better properties would improve the efficiency, heat resistance, and service life of organic EL elements. Now, in the majority of organic El elements, light is emitted mostly from the light-emitting layer that is provided separately from the charge-transporting layer or from the electron-transporting layer and rarely from the hole-transporting layer. This may be due partly to the problem of compatibility of the hole-transporting layer with the simultaneously used electron-transporting layer, but a factor just as important appears to be the color and intensity of the light emitted from the hole-transporting material itself. The hole-transporting layer is expected to increase in practical value if light can be emitted therefrom, but materials useful for this purpose are few. As represented by the 9-anthranyl derivatives described in JP9-157643 A, the materials of this kind in many cases have faced problems such as emission of light of long wavelength and no emission of light of short wavelength.